It's the 'careful layout of the holes' that's giving me problems. I
guess I could do it by using the existing router base as a template,
because it would be before cutting the central hole.

Tell me more about the polycarbonate (is that Lexan?) scraps. What
sort of thickness are they? Would they be suitable for a table base
(strong enough to not sag)?

I assumed that you had basic measuring equipment. The screw hole layout
is usually an even multiple, ie. 4.5" or 250 mm. I locate where I want
the center hole and scribe an accurate circle that will pass through
the center of the mounting holes. Then I scribe the 'bolt' circle with
dividers by trial and error until the holes are perfectly spaced, 3 or
4 depending on the router. Once the centers are located, carefully
prick punch them and align under the drill press with a pointed
centering bit. Then drill and countersink as required. This is a pretty
basic exercise in hand layout which any apprentice has to learn.
I use 3/8" Lexan for my bases. That's what the expensive router bases
are made from, but you can't always depend on an accurate mounting hole
layout from them.
Using it for a table depends entirely on the size of the table. Lexan
is plastic and will flex slightly on a large span.

Bugs